Polymer electrolyte comprising inorganic conductive nano-particles and fuel cell employing the polymer electrolyte

ABSTRACT

A polymer electrolyte and a fuel cell employing the polymer electrolyte are provided. The polymer electrolyte includes: an ionic conductive polymer membrane; a porous support having nano-sized pores; and inorganic conductive nano-particles including an ionic conductive material impregnated into the porous support, wherein the inorganic conductive nano-particles are impregnated into microchannels formed by aggregation of polar portions of the ionic conductive polymer membrane, and/or between polymer backbones of the ionic conductive polymer membrane.

Priority is claimed to Patent Application Number 2001-67148 filed inRepublic of Korea on Oct. 30, 2001, herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polymer electrolyte containinginorganic conductive nano-particles and a fuel cell employing thepolymer electrolyte, and more particularly, to a polymer electrolytehaving excellent ionic conductivity and capable of effectively blockingflow of methanol by inorganic conductive nano-particles contained in thepolymer electrolyte and a fuel cell employing the polymer electrolyte.

2. Description of the Related Art

Direct methanol fuel cells (DMFCs) directly supply a liquid fuel such asmethanol to electrodes, so they need no additional equipment such as afuel reformer or a hydrogen reservoir, which is required in polymerelectrolyte fuel cells. DMFCs can be miniaturized to be readily appliedunder any urgent situation. Furthermore, DMFCs have a high energydensity and is easily available as an environment-friendly, clean energysource. One of most important factors which affect the performance andcost of such DMFCs is a polymer electrolyte used therein.

In a fluorinated polymer membrane, which is widely used as a polymermembrane for a DMFC, methanol is liable to migrate while being hydrated,rather than be oxidized, so that cross-over of the methanol through thepolymer membrane occurs. Greater cross-over of the methanol furtherreduces the potential of the fuel cell and hinders the reduction ofoxygen and thus is considered to be the main cause of performancedegradation of the fuel cell. Therefore, suppressing the cross-over ofmethanol is the first consideration for practical uses of the DMFC.

The DMFC needs a kind of barrier for blocking direct migration of themethanol between an anode and an cathode while allowing only protons topass.

Even if a polymer membrane capable of acting as a barrier that passesonly protons while suppressing the cross-over of the methanol isavailable, performance of the DMFC may degrade due to generation ofover-voltage if proton conductivity of the polymer membrane is low.

U.S. Pat. No. 5,874,182 discloses a method of minimizing the effect ofthe cross-over of the methanol by using excess catalyst in theelectrode. However, this method was unsatisfactory in suppressing themethanol's cross-over and is not considered to be a fundamental solutionof the cross-over. As a result, many attempts have been made to improvethe performance of the DMFC by improving the polymer electrolytemembrane itself so as to prevent the cross-over of methanol.

U.S. Pat. No. 5,795,668 discloses a method of reducing cross-over ofmethanol by supporting solid polymer electrolyte membranes against bothsides of a porous support. U.S. Pat. No. 5,958,616 discloses a method ofoxidizing methanol, with the addition of a catalyst, to an electrolytemembrane that is formed to incorporate channel or path for methanoltherein. However, this method increased the cross-over of methanolthrough the channel or path of the electrolyte membrane. In addition,the use of the catalyst and the porous support further increases thecost of the solid electrolyte membrane, which is already expensive toproduce, and thus the method is impractical.

U.S. Pat. No. 5,919,583 discloses a method of reducing cross-over in aDMFC by dispersing an inorganic material such as zeolite and zirconiumin the polymer electrolyte. However, this method has the followingproblems. To incorporate such inorganic particles into the polymerelectrolyte composition, there is a need to change the polymerelectrolyte composition into an alkaline state, which is a complicatedprocess. Furthermore, such simple dispersion of the inorganic particlesin the polymer electrolyte membrane is effective in preventing themethanol crossover but the proton conductivity is decrease. As describedabove, although approaches have been made in a variety of aspects toreduce the cross-over of methanol in the DMFC, those suggestions stillhave considerable technical limitations in the decrease protonconductivity.

SUMMARY OF THE INVENTION

To solve the above-described problems, it is a first object of thepresent invention to provide a polymer electrolyte having excellentionic conductivity and capable of effectively reducing cross-over ofmethanol by inorganic conductive nano-particles dispersed therein and amethod for preparing the polymer electrolyte.

It is a second object of the present invention to provide a fuel cellhaving improved efficiency by employing the polymer electrolyte.

To achieve the first object of the present invention, there is provideda polymer electrolyte comprising: an ionic conductive polymer membrane;a porous support having nano-sized pores; and inorganic conductivenano-particles including an ionic conductive material impregnated intothe porous support, wherein the inorganic conductive nano-particles areimpregnated into at least one selected from the group consisting ofmicrochannels formed by aggregation of polar portions of the ionicconductive polymer membrane, and between polymer backbones of the ionicconductive polymer membrane.

Preferably, the ionic conductive material is at least one selected fromthe group consisting of a heteropoly acid of formula (1) below, aphosphoric acid of formula (2) below, a sulfuric acid, and salts ofthese materials:

where X is one selected from the group consisting of boron (B), aluminum(Al), gallium (Ga), tin (Sn), phosphorous (P), antimony (Sb), tellurium(Te), iodine (I), and transition metals; Y is a transition metal; m1 isan integer from 1 to 10; n1 is an integer from 2 to 100, R is selectedfrom the group consisting of C1-C20 hydroxyalkyl, C1-C20 alkyl, phenyl,phenyl substituted with C1-C20 alkyl group, vinyl, C1-C20 halogenatedalkyl, halogenated phenyl, halogenated methylphenyl, and amine groups;m2 is an integer from 1 to 10; and n2 is an integer from 0 to 20.

Preferably, each of the transition metals X and Y is one selected fromthe group consisting of tungsten (W), molybdenum (Mo), phosphorous (P),silicon (Si), cobalt (Co), cesium (Cs), vanadium (V), and nickel (Ni).

Preferably, the inorganic conductive nano-particles have a size of fromabout 3 to about 50 nm. Preferably, the ionic conductive polymermembrane comprises at least one ionic conductive polymer selected fromthe group consisting of a 4-fluorinated sulfonated polymer and a benzenesulfonated polymer membrane having a benzene ring, and the ionicconductive polymer membrane has a thickness of from 30 to 200 μm.

Preferably, the porous support is at least one selected from the groupconsisting of silica, alumina, zirconia, zeolite, and titania, and theporous support has a pore size of from 0.1 to 50 nm.

In the polymer electrolyte according to the present invention,preferably, an amount of the porous support is in the range of 10-90parts by weight based on 100 parts by weight the inorganic conductivenano-particles, an amount of the ionic conductive material is in therange of 10-90 parts by weight based on the inorganic conductivenano-particles, and an amount of the ionic conductive nano-particles isin the range of 3-90 parts by weight based on 100 parts by weight thepolymer electrolyte.

To achieve the first object of the present invention, there is alsoprovided a method for preparing a polymer electrolyte, the methodcomprising: (a) obtaining a mixture in an oily phase by adding a solventto a surfactant; (b) soaking an ionic conductive polymer membrane in themixture, stirring and adding an ionic conductive material and pure waterto the mixture, and neutralizing the mixture with a base; (c) adding aprecursor of a porous support to the mixture and mixing the mixture; (d)drawing the ionic conductive polymer membrane out of the mixture andwashing and drying the ionic conductive polymer membrane, whereininorganic conductive nano-particles are impregnated into at least oneselected from the group consisting of micro-channels, and polymerbackbone of the ionic conductive polymer membrane, the micro-channelsbeing formed by agglomeration of polar portions of an ionic conductivepolymer constituting the ionic conductive polymer membrane.

Preferably, the solvent in step (a) is at least one selected from thegroup consisting of isooctane, n-hexane, cyclohexane, dodecane, toluene,decane, heptane, hexadecane, and 1-hexanol, and an amount of the solventis in the range of 8000 to 12000 parts by weight based on 100 parts byweight the ionic conductive material.

Preferably, the surfactant in step (a) comprises a non-ionic surfactantselected from the group consisting of Triton (benzyltrimethylammoniumhydroxide), Berol 26 (polyoxyethylene nonylphenylether), and Berol 160(polyoxyethylene dodecylether), and an ionic surfactant selected fromthe group consisting of SPAN (sorbitan monoleate, sorbitanmonopalmitate), Arlacel (sorbitan sesquioleate), and AOT (sodiumbis(2-ethylhexyl)sulfoxylmate). Preferably, an amount of the surfactantis in the range of 50-150 parts by weight based on 100 parts by weightthe ionic conductive material. Preferably, an amount of the pure waterin step (b) is in the range of 3-10 parts by weight based on 100 partsby weight the ionic conductive polymer constituting the ionic conductivepolymer membrane, and the base in step (b) is one selected from thegroup consisting of ammonia water and sodium hydroxide.

Preferably, the precursor of the porous support in step (c) is at leastone selected from the group consisting of zirconium alkoxide, titaniumalkoxide, silicon alkoxide, and aluminum alkoxide, and an amount of theprecursor is in the range of 5-90 parts by weight based on 100 parts byweight the ionic conductive material.

Preferably, an alcoholic solvent is used as a washing solvent in step(d), and the drying is performed at a temperature of 80-120° C.

The second object of the present invention is achieved by a fuel celladopting the polymer electrolyte described above. Preferably, the fuelcell according to the present invention is a direct methanol fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will becomemore apparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIG. 1 illustrates the impregnation of inorganic conductivenano-particles into a micro-channel and between polymer backbones of anionic conductive polymer according to the present invention;

FIG. 2 is a graph of ionic conductivity of a polymer electrolytemanufactured in Example 3 according to the present invention;

FIG. 3 is a graph of a change in cell potential with respect to currentdensity in direct methanol fuel cells (DMFCs) manufactured in Example 12and Comparative Examples 1 and 2 according to the present invention;

FIG. 4 is a graph of the cross-over of methanol in DMFCs manufactured inExample 12 according to the present invention and Comparative Example 2;and

FIG. 5 is a graph of distribution of silica nano-particle sizes preparedin Example 3 according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Prior to description of the principle of the present invention, the termof “micro-channel” used throughout this specification will be defined. A“micro-channel” refers to a kind of path formed by aggregation of polarportions of a conductive polymer membrane, which consists of the polarportions and non-polar portions.

A polymer electrolyte according to the present invention has a structurewhere inorganic conductive nano-particles are impregnated intomicro-channels and/or between polymer backbones of a conductive polymermembrane constituting the electrolyte. An embodiment of the structure ofthe polymer electrolyte according to the present invention will bedescribed in greater detail with reference to FIG. 1.

FIG. 1 shows a fluorocarbon ionomer as an ionic conductive polymer.Referring to FIG. 1, the ionomer roughly divided into a non-polarportion and a polar portion. Fluorocarbon backbone A is considered to bethe non-polar region. Free volume inside fluorocarbon backbone A isexpanded when the polymer electrolyte is hydrated in water or alcoholicsolution. In the polar portion where negative and positive ions of theterminal of the backbone are also expanded, if the polymer electrolyteis hydrated in water or alcoholic solution. As shown in FIG. 1, polarportions (cluster portion C) are connected to form a micro-channel, andinorganic conductive nano-particles are impregnated into themicro-channel and between polymer backbones. In FIG. 1, referencecharacter B denotes a boundary portion.

A method for preparing the polymer electrolyte according to the presentinvention will be described. Initially, a surfactant is dispersed into asolvent to obtain a mixture in an oily phase. The surfactant may be anon-ionic surfactant selected from the group consisting of Triton(benzyltrimethylammonium hydroxide), Berol 26 (polyoxyethylenenoonylphenylether), Berol 160 (polyoxyethylene dodecylether), and Bero267 (polyoxyethylene nonylphenyl ether), or an ionic surfactant selectedfrom the group consisting of SPAN (sorbitan monoleate, sorbitanmonopalmitate), Arlacel (sorbitan sesquioleate), and AOT (sodiumbis(2-ethylhexyl)sulfoxylmate). The surfactant enables emulsionformation. An amount of the surfactant is in the range of 100-200 partsby weight based on 100 parts by weight an ionic conductive material. Asolvent as a dispersion medium of the surfactant is at least oneselected from the group consisting of isooctane, n-hexane, cyclohexane,dodecane, toluene, decane, heptane, hexadecane, and 1-hexanol. Thesolvent enables oily phase formation. An amount of the solvent is in therange of 8000-12000 parts by weight based on 100 parts by weight theionic conductive material. If an amount of the solvent exceeds thisrange, it may be difficult to obtain particles of a desired size. Also,due to agglomeration of the particles, the particles are enlarged to onthe order of several microns and become non-uniform in size.

Next, an ionic conductive polymer membrane is soaked in the oily phasemixture and thoroughly mixed. Here, as the ionic, conductive polymermembrane, a 4-fluorinated sulfonated polymer membrane or a benzenering-attached sulfonated polymer membrane is used. Suitable4-fluorinated sulfonated polymer membranes include a Nafion 112 membrane(DuPont), a Nafion 115 membrane (DuPont), Aciplex membrane (Ashai GlassCo.), a Gore membrane, a perfluorinated sulfonic acid membrane, asulfonated polystyrene membrane, a sulfonated polyimide membrane, apolybenzimidazole membrane, and a sulfonated polyethersulfone membrane.Suitable benzene ring-attached sulfonated polymer membranes include abenzene ring-attached polyether sulfone membrane, a benzenering-attached polysulfone membrane, and a benzene ring-attachedpolystyrene membrane. Here, it is preferable that the ionic conductivepolymer membrane has a thickness of 30-200 μm. If a thickness of theionic conductive polymer membrane is less than 30 μm, the ionicconductive polymer membrane may be easily broken when inorganicparticles are impregnated thereinto. If a thickness of the ionicconductive polymer membrane is greater than 200 μm, resistance mayincrease to reduce ionic conductivity.

Next, after addition of an ionic conductive material and pure water, themixture is neutralized with alkali. Here, as the ionic conductivematerial, at least one selected from the group consisting of aheteropoly acid of formula (1) below, a phosphoric acid of formula (2)below, sulfuric acid and salts of these materials are used. It ispreferable that an amount of the ionic conductive material is in therange of 10-90 parts by weight based on 100 parts by weight inorganicconductive nano-particles. If an amount of the ionic conductive materialexceeds this range, it is undesirable in terms of conductivity of theinorganic conductive nano-particles.

where X is one selected from the group consisting of boron (B), aluminum(Al), gallium (Ga), tin (Sn), phosphorous (P), antimony (Sb), tellurium(Te), iodine (I), and transition metals; Y is a transition metal; m1 isan integer from 1 to 10; n1 is an integer from 2 to 100, R is selectedfrom the group consisting of C1-C20 hydroxyalkyl, C1-C20 alkyl, phenyl,phenyl substituted with C1-C20 alkyl group, vinyl, C1-C20 halogenatedalkyl, halogenated phenyl, halogenated methylphenyl, and amine groups;m2 is an integer from 1 to 10; and n2 is an integer from 0 to 20.

In formula (1) and (2) above, examples of the transition metal includetungsten (W), molybdenum (Mo), phosphorous (P), silicon (Si), cobalt(Co), cesium (Cs), vanadium (V), and nickel (Ni). Examples of the C1-C20hydroxyalkyl group includes hydroxymethyl and hydroxyethyl groups.Examples of the C1-C20 alkyl group include methyl, ethyl, propyl, butyl,and pentyl groups. Examples of the phenyl group substituted with C1-C20alkyl group include methylphenyl and ethylphenyl groups. Examples of theC1-C20 halogenated alkyl group include chloromethyl and chloroethylgroups. An example of the halogenated phenyl group includes achlorophenyl group. An example of the halogenated methylphenyl groupincludes a chloromethylphenyl group.

Preferably, the heteropoly acid of formula (1) above comprises 12tungsto (VI) phosphoric acid, silicotungsto (VI) phorphoric acid,tungstosilicic acid, cesium hydrogen tungstosilicate, molybdophosphoricacid, molybdosilicic acid, ammonium molybdodiphosphate, sodiummolybdophosphate, potassium tungstophosphate, and potassiummolybdodivanado phosphate.

Any base capable of neutralizing the reaction mixture can be used.Examples of the base include ammonia water and sodium hydroxide (NaOH).An amount of the base is adjusted such that the pH of the reactionmixture is maintained at 5-7. An amount of the pure water added is inthe range of 60-70 parts by weight based on 100 parts by weight theionic conductive material. If an amount of the pure water added exceedsthis range, agglomeration of the particles occurs.

Next, a precursor of a porous support is added to the neutralizedreaction mixture prepared above and stirred such that inorganicconductive nano-particles are impregnated into microchannels of an ionicconductive polymer membrane, and/or between polymer backbones of theionic conductive polymer membrane. The precursor of the porous supportis selected from the group consisting of zirconium alkoxide, titaniumalkoxide, silicon alkoxide, and aluminum alkoxide. An amount of theprecursor is in the range of 50-300 parts by weight based on 100 partsby weight the ionic conductive material. If an amount of the poroussupport precursor exceeds this range, the resulting particles may begreater than a desired particle size.

Next, the resulting ionic conductive polymer membrane is drawn out ofthe mixture and washed to remove the surfactant. An alcoholic solventsuch as ethanol is used as a washing solvent.

Next, the ionic conductive polymer membrane is dried to obtain a polymerelectrolyte containing the inorganic conductive nano-particlesimpregnated into the microchannels of the ionic conductive polymer,and/or between polymer backbones of the ionic conductive polymermembrane. The drying process is performed at a temperature of 80-120°C., but may be varied depending on the type of the ionic conductivepolymer used.

The polymer electrolyte according to the present invention preparedthrough the processes described above includes the ionic conductivepolymer membrane and the inorganic conductive nano-particles containingthe porous support having nano-sized pores and the ionic conductivematerials impregnated into the porous support. The porous support mayhave a pore size of 0.1-300 nm and may be formed of at least oneselected from the group consisting of silica, alumina, zirconia,zeolite, and titania. Preferably, an amount of the porous support may bein the range of 50-300 parts by weight based on 100 parts by weight theinorganic conductive nano-particles. If a pore size and amount of theporous support exceed those ranges, it may be difficult to stablysupport the ionic conductive materials with the porous support.

In the present invention, it is preferably that the inorganic conductivenano-particles have a size of 0.1-50 nm. If a size of the inorganicconductive nano-particles is greater than 50 nm, it is impossible toimpregnate the inorganic conductive nano-particles into themicrochannels of the polymer membrane. If a size of the inorganicconductive nano-particles is smaller than 0.1 nm, the inorganicconductive nano-particles are undesirably separated from the polymermembrane over time. Preferably, an amount of the inorganic conductivenano-particles impregnated into the polymer membrane is in the range of3-90 parts by weight based on 100 parts by weight the polymerelectrolyte. If an amount of the inorganic conductive nano-particles isgreater than 90 parts by weight, formation of the polymer membrane maybe difficult. If an amount of the inorganic conductive nano-particles isless than 3 parts by weight, the addition of the inorganic conductivenano-particles is not effective.

A fuel cell according to the present invention is manufactured byforming a single cell by placing between the anode and cathode thepolymer electrolyte prepared through the processes above. The fuel cellincludes proton exchange membrane fuel cells (PEMFCs) and directmethanol fuel cells (DMFCs). When a DMFC adopts the polymer electrolyteaccording to the present invention, a higher inorganic conductivenano-particle content in the polymer membrane more effectively reducesthe cross-over of methanol, thereby improving cell efficiency.

The polymer electrolyte according to the present invention is applicableto fuel cells, sensors, electrochemical displays, etc. When used as anelectrolyte membrane of a fuel cell, it provides good ionic conductivityand effectively reduces the cross-over of methanol so that cellefficiency is improved.

The present invention will be described in greater detail with referenceto the following examples. The following examples are for illustrativepurposes and are not intended to limit the scope of the invention.

Example 1 Preparation of Polymer Electrolyte

7.12 g dioctyl sulfosuccinate was completely dissolved in 300 gcyclohexane and 60 g 1-hexanol to form an oily phase. A Nafion 115polymer membrane (Aldrich Chemical Co.) was put into the solution andstirred. After additions of 3 g 12 tungsto (VI) phosphoric acid(H₃PW₁₂O₄₀nH₂O, where n=6-8) and 2 g pure water and mixing, the mixturewas pH-adjusted to be 5-7 with 1 g of ammonia water and stirred for 30minutes.

Subsequently, 10 g tetraethyl orthosilicate was added to the mixture andstirred for 36-49 hours. The membrane was drawn out of the mixture,washed with ethanol to remove the surfactant, and dried at 90° C. toobtain a polymer electrolyte containing 12 tungsto (VI) phosphosphoricacid-impregnated silica nano-particles impregnated into microchannels ofthe Nafion 115 membrane, and between polymer backbones of the Nafion 115membrane.

Example 2

A polymer electrolyte was prepared in the same manner as in Example 1except that 1 g dioctyl sulfosuccinate, 28.55 g 12 tungsto (VI)phosphoric acid (H₃PW₁₂O₄₀nH₂O, where n=6-8), and 41.60 g tetraethylorthosilicate were used.

Example 3

A polymer electrolyte was prepared in the same manner as in Example 2except that 3 g 12 tungsto (VI) phosphoric acid (H₃PW₁₂O₄₀nH₂O, wheren=6-8) and 0.14 g ammonia water were used.

Example 4

A polymer electrolyte was prepared in the same manner as in Example 3except that 36 g pure water and 10 g tetraethyl orthosilicate were used.

Example 5

A polymer electrolyte was prepared in the same manner as in Example 4except that 28.55 g 12 tungsto (VI) phosphoric acid (H₃PW₁₂O₄₀nH₂O,where n=6-8) and 1 g ammonia water were used.

Example 6

A polymer electrolyte was prepared in the same manner as in Example 5except that 3 g 12 tungsto (VI) phosphoric acid (H₃PW₁₂O₄₀nH₂O, wheren=6-8) and 41.60 g tetraethyl orthosilicate were used.

Example 7

A polymer electrolyte was prepared in the same manner as in Example 6except that 28.55 g 12 tungsto (VI) phosphoric acid (H₃PW₁₂O₄₀nH₂O,where n=6-8) and 0.14 g ammonia water were used.

Example 8

A polymer electrolyte was prepared in the same manner as in Example 7except that 2 g pure water and 10 g tetraethyl orthosilicate were used.

For the polymer electrolytes prepared in Examples 1 through 8, the sizedistribution of the silica nano-particles was measured. Here, the sizeof silica nano-particles were measured using a MICROTRAC-UPA150(Honeywell Co) and from the degree of scattering of laser light in adispersion of the inorganic particles.

As a result, the inorganic conductive nano-particles of Examples 1through 8 were observed to have a particle size ranging from 5 to 200 nmon average. FIG. 5 shows the size distribution of the silicanano-particles prepared in Example 3. Referring to FIG. 5, the averageparticle size is about 5 nm. In FIG. 5, “% PASS” on the left Y-axisdenotes the percentage of particles smaller than a given size, and “%CHAN” on the right Y-axis denotes the percentage with a given size.

The amount of tungsten (W) in each of the polymer electrolytes ofExamples 1 through 8 and the Nafion 115 membrane was analyzed usinginductively coupled plasma (ICP). The results are shown in Table 1.

TABLE 1 Example W content (wt %) Example 1 0.085 Example 2 0.444 Example3 0.796 Example 4 0.078 Example 5 0.084 Example 6 0.025 Example 7 0.431Example 8 0.082 Nafion 115 membrane 0.0080

As shown in Table 1, considerable increases in the amounts of W areobserved in the polymer electrolytes of Examples 1 through 8, comparedto the Nafion 115 membrane. This result supports the fact that silicanano-particles are impregnated into the Nafion 115 membrane of thepolymer electrolytes.

The ionic conductivity of the polymer electrolyte prepared in Example 3was measured. The result is shown in FIG. 2. As shown in FIG. 2, theelectrolyte polymer of Example 3 has excellent ionic conductivity.

Likewise, the ionic conductivity of the polymer electrolytes of Examples1 and 2, and 4 through 8 was measured. Those polymer electrolytes have asimilar ionic conductivity to the electrolyte polymer of Example 3.

Example 9 Manufacture of Direct Methanol Fuel Cell

A polymer electrolyte prepared according to Example 1 was washed withdistilled water and dried at 95° C. On both sides of the dried polymerelectrolyte, a PtRu anode and a Pt cathode were arranged to form a unitcell so that a DMFC was manufactured using the unit cell.

Examples 10 Through 19

DMFCs were manufactured in the same manner as in Example 9 except thatthe polymer electrolytes of Examples 2 through 8 were used,respectively, instead of the polymer electrolyte of Example 1.

Comparative Example 1

A DMFC was manufactured in the same manner as in Example 9 except that aNafion 112 membrane was used instead of the polymer electrolyte ofExample 1.

Comparative Example 2

A DMFC was manufactured in the same manner as in Example 9 except that aNafion 115 membrane was used instead of the polymer electrolyte ofExample 1.

Cell efficiency was measured for the DMFCs manufactured in Example 12and Comparative Examples 1 and 2. The results are shown in FIG. 3. Here,cell efficiency was measured as a current density with respect topotential and a ratio of current density reduction due to the cross-overof methanol. The greater the current density with respect to potentialand the smaller the current density reduction due to the methanol'scross-over, the better the cell efficiency. The results are shown inFIGS. 3 and 4.

Referring to FIG. 3, the DMFC of Example 12 has better or similar cellperformance than Comparative Examples 1 and 2. The current density withrespect to cell potential is higher for the DMFC of Example 12 than forthe DMFCs of Comparative Examples 1 and 2. The current density of theDMFC of Example 12 was measured with different amounts of nano-particlesat 5 wt % by weight and 30 wt % by weight. The amount of thenano-particles was calculated by measuring the weight of the Nafion 115membrane before and after soaking.

In FIG. 4, the Y-axis denotes the amount of reduction in the currentdensity due to cross-over of methanol. The current density loss in theDMFC using the Nafion 115 membrane (Comparative Example 2) was set to 1,and the amount of reduction in the current density of the DMFC ofExample 12 was measured relatively with respect to Comparative Example2. As is apparent from FIG. 4, the current density of the DMFC ofExample 12 is less reduced than that of the DMFC of Comparative Example2 using the Nafion 115 membrane. It is evident from this result that anamount of the methanol's cross-over is reduced in the DMFC of Example 12than in the DMFC using the Nafion 115 membrane.

In the polymer electrolyte according to the present invention, since theinorganic conductive nano-particles are impregnated into themicrochannels of the ionic conductive polymer membrane and/or betweenpolymer backbones, a flow path of methanol become small and thus thecross-over of the methanol is reduced. The polymer electrolyte accordingto the present invention has good ionic conductivity. A fuel celladopting the polymer electrolyte according to the present invention alsocan effectively reduce the cross-over of methanol and has high protonconductivity to increase the proton transmission rate. Accordingly, thefuel cell using the polymer electrolyte is remarkably improved in cellefficiency.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A polymer electrolyte formed by the method comprising: (a) obtaininga mixture in an oily phase by adding a solvent to a surfactant; (b)soaking an ionic conductive polymer membrane in the mixture, stirringand adding an ionic conductive material and pure water to the mixture,and neutralizing the mixture with a base; (c) adding a precursor of aporous support to the mixture and mixing the mixture; and (d) drawingthe ionic conductive polymer membrane out of the mixture and washing anddrying the ionic conductive polymer membrane, wherein inorganicconductive nano-particles are impregnated into at least one selectedfrom the group consisting of micro-channels of the ionic conductivepolymer membrane, and polymer backbone of the ionic conductive polymermembrane the micro-channels being formed by agglomeration of polarportions of an ionic conductive polymer constituting the ionicconductive polymer membrane.
 2. The polymer electrolyte of claim 1,wherein the ionic conductive material is at least one selected from thegroup consisting of a heteropoly acid of formula (1) below, a phosphoricacid of formula (2) below, a sulfuric acid, and salts of thesematerials:

where X is one selected from the group consisting of boron (B), aluminum(Al), gallium (Ga), tin (Sn), phosphorous (P), antimony (Sb), tellurium(Te), iodine (I), and transition metals; Y is a transition metal; m1 isan integer from 1 to 10; n1 is an integer from 2 to 100, R is selectedfrom the group consisting of C1-C20 hydroxyalkyl, C1-C20 alkyl, phenyl,phenyl substituted with C1-C20 alkyl group, vinyl, C1-C20 halogenatedalkyl, halogenated phenyl, halogenated methylphenyl, and amine groups;m2 is an integer from 1 to 10; and n2 is an integer from 0 to
 20. 3. Thepolymer electrolyte of claim 1, wherein the solvent in step (a) is atleast one selected from the group consisting of isooctane, n-hexane,cyclohexane, dodecane, toluene, decane, heptane, hexadecane, and1-hexanol, and an amount of the solvent is in the range of 8000 to 12000parts by weight based on 100 parts by weight the ionic conductivematerial.
 4. The polymer electrolyte of claim 1, wherein the surfactantin step (a) comprises a non-ionic surfactant selected from the groupconsisting of benzyltrimethylammonium hydroxide, polyoxyethylenenonylphenylether, and polyoxyethylene dodecylether, and an ionicsurfactant selected from the group consisting of dioctyl sulfosuccinate,sorbitan monoleate, sorbitan monopalmitate, sorbitan sesquioleate, andsodium bis(2-ethylhexyl)sulfoxylmate.
 5. The polymer electrolyte ofclaim 1, wherein an amount of the pure water in step (b) is in the rangeof 1-10 parts by weight based on 100 parts by weight the ionicconductive polymer constituting the ionic conductive polymer membrane,and the base in step (b) is one selected from the group consisting ofammonia water and sodium hydroxide.
 6. The polymer electrolyte of claim1, wherein the precursor of the porous support in step (c) is at leastone selected from the group consisting of zirconium alkoxide, titaniumalkoxide, silicon alkoxide, and aluminum alkoxide, and an amount of theprecursor is in the range of 5-90 parts by weight based on 100 parts byweight the ionic conductive material.
 7. The polymer electrolyte ofclaim 1, wherein an alcoholic solvent is used as a washing solvent instep (d), and the drying is performed at a temperature of 80-120.